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For Clues on Climate, Seeing What Packrats Kept

Geoffrey Spaulding and Kenneth L. Cole lifted off from a high plateau in the Grand Canyon, their helicopter laden with so many packrat nests that it could barely climb.

“The chopper gave this sickly shudder as we made our way back across the chasm of the canyon,” Dr. Spaulding recalled. “Thank goodness for those Vietnam-vet pilots.”

To Dr. Spaulding, a geologist with the engineering company CH2M Hill, and Dr. Cole, an ecologist for the United States Geological Survey, the nests were precious cargo. Packrats, which look like brown squirrels with Dumbo ears, are skilled home builders, and their massive nests, known as middens, can last 10,000 to 20,000 years (though they are not usually inhabited the entire time).

For that reason, the middens serve as time capsules of desert ecology. By analyzing preserved ancient plants and scat from a variety of middens dating back 12,000 years, Dr. Cole recently proved that a miniature ice age known as the Younger Dryas, long thought to have been confined to the North Atlantic, was also felt in the American Southwest.

The analysis demonstrated that after the Younger Dryas, average temperatures in the area climbed about 14 degrees Fahrenheit in just a few hundred years — a precipitous rise that is lending insight into the effects of today’s warming trends on desert ecosystems.

“After the warming period, you notice that fewer tree and shrub species appear in the middens,” Dr. Cole said. “That’s exactly what’s happening in the Southwest now.”

Photo

John Cannella poking at a packrat nest, known as a midden. Credit
Kenneth L. Cole/U.S.G.S.

Through carbon dating of specimens from packrat nests, scientists can put together a portrait of the prevalence of different plant species in an area over time, similar to the long view obtained by drilling into Arctic ice cores for plant samples trapped there.

“There are twigs and seeds and things in the middens that are easy to work with,” Dr. Cole said. “You don’t have to look at some microscopic piece of leaf. The rats collect some of everything. The nest is like a snapshot of that particular spot at some time in the past.”

The packrats’ middens, hulking structures up to 10 feet wide, serve as trash heaps, climate-controlled homes and defensive ramparts. “The rats leave piles of debris in front of the entrance to preserve moisture,” Dr. Cole said, “and the debris helps seal up the crevice so larger animals can’t get in.”

To turn grasses and leaf bits into sturdy walls, the rats use the only natural glue available: their own urine. “Packrats don’t drink water, so their urine is very viscous,” Dr. Cole said. “It crystallizes and becomes solid.” Like hardened amber, the solidified waste is an ideal matrix to keep plant fragments intact for thousands of years.

When researchers find a new nest in a cave or an outcropping, they pick it apart like prospectors — an extremely smelly process — and analyze the plants they find for carbon-isotope ratios that identify them as members of a particular species. Some plants, like junipers, usually grow relatively high in the mountains, while water-retaining species like agave and prickly pear are found at lower elevations.

If a climate shift occurs, changes in these norms are recorded in the contents of middens at different altitudes. During cooler periods, for instance, most vegetation migrates downhill. The agave can ordinarily be found near cliff edges, but “during the Younger Dryas cold snap, it was not growing at higher elevations than Lake Mead,” Dr. Cole said. As warming occurs, the plants move back uphill; some even disappear from the middens altogether.

Because no layers of sea or ice sediments can be found in most dry inland regions, plants from the middens have been scientists’ only way of piecing together climactic records for much of the Western United States.

“The limitation of the midden method is that in most cases, it’s hard to get samples from the same site spanning a long period of time,” Dr. Spaulding said. “You go to a cliff face and collect data from one midden, then look at a second and third one a few miles away, each with subtle differences in ecology. So when we see changes in plant life from one of these middens to the next, it doesn’t always indicate a climate shift.”

Still, researchers have collected enough data to correct quite a few misconceptions. “Before scientists started studying these middens, people assumed the Southwestern desert had been in place for millions of years,” Dr. Spaulding said. “If we hadn’t done this work, we’d still be in a Stone Age of ecology when it comes to this region.”

Middens contain markers of other types of environmental change as well. After dissolving urine from 10,000-year-old nests near Yucca Mountain, Nev., Mitch Plummer, a hydrogeologist at the Idaho National Laboratory, found that the samples had surprisingly high levels of chlorine 36 isotopes.

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Produced when stratospheric atoms are bombarded by cosmic rays, chlorine 36 washes into the lower atmosphere in the form of rain, which packrats ingest when it falls on desert plants. One possible explanation for the high levels is that the jet stream, where most blending of upper and lower atmospheric layers occurs, moved south during this time.

“The shifting of the jet stream changes the way chlorine 36 mixes in the atmosphere, and we think that’s what contributed to these high concentrations,” Dr. Plummer said.

Taken as a whole, data from ancient packrat nests, particularly those from the rapid warming period right after the Younger Dryas, hold some ominous signs for the prospects of desert environments.

“Plants responded rapidly to climate change if they were able to move uphill, but plants migrating northward took up to 10,000 years to adapt,” Dr. Cole said. “The rate of warming now is very similar to what it was then. What we see happening in the middens is happening to the world.”

A version of this article appears in print on , on Page F3 of the New York edition with the headline: For Clues On Climate, Seeing What Packrats Kept. Order Reprints|Today's Paper|Subscribe